Kids Behaving Badly? Blame It on Mom

All little kids can be aggressive, but those who remain explosive by the time they enter kindergarten have their mothers to blame, according to new research published Wednesday in the journal Child Development.

Some children are clearly more temperamental than others. Researchers have wondered whether it was nature at work, or nurture. Or a combination? Scientists at the University of Minnesota delved into the puzzle, scrutinizing 267 mother-child pairs, or dyads, recruited from a local public-health clinic. Mom will be none too pleased with the conclusion: a hostile relationship between mothers and their little ones gives rise to persistent defiant behavior.

In general, babies are fairly placid. But somewhere in late infancy and early toddlerhood, kids get to be a handful. (You’re probably familiar with the Terrible Twos.) In a typical developmental trajectory, aggression is supposed to peak at age 2.5, then subside. In theory, a typical 4-year-old is a lot less aggressive than a typical 2-year-old, and a typical 6-year-old is better-behaved than your average 4-year-old.

But there are always some behavioral outliers. “Kids at the top of the pile who are real hellions in kindergarten or first grade have a pronounced risk of staying that way,” says lead author Michael Lorber, a research scientist in the Family Translational Research Group at New York University who was formerly at the University of Minnesota. “If a 5-year-old acts like a 2-year-old, you are in trouble.”

Childhood aggression — hitting, for example, or throwing frequent tantrums — has been associated with problems in school, depression, drinking and, down the road, aggressive behavior toward a spouse.

In the average American family, aggression is often modeled by parents. Lorber’s research group has found that more than 70% of moms and dads hit or spank their children. But that’s not the point of this particular study, which subjected 7-to-10-day-old babies to a neurological exam that included pinpricks and cold objects placed against their skin. The infants’ reaction to the annoying sensations determined their baseline level of “neonatal difficulty.” Then, when the infants were 3 and 6 months old, researchers asked mothers about their babies’ temperament. At the same time, they observed the mothers feeding their children, noting whether the women acted affectionate or irritated.

Later, at 24 and 42 months, the mothers were observed assisting the children with extracting objects from a tube. Finally, when the children were in kindergarten or first grade, mothers and teachers each rated the children’s behavior.

The biggest predictor of sustained hostility? Negative parenting at three and six months. “Nothing about the child’s behavior in infancy predicted anything,” says Lorber. “But we did find that negative parenting in infancy was really important.”

So what to do? For starters, don’t beat yourself up (figuratively speaking, of course) if you’re a mom of a rambunctious grade-schooler. It’s likely that mother-infant interaction isn’t the only factor at play here. Genetics is probably influencing behavior as well.

Second, be nice to your kids. “Our research suggests you need to start really early,” says Lorber. “Being a sensitive parent and responding to your baby’s social and emotional needs is always a good thing.”

Bonnie Rochman is a reporter at TIME. Find her on Twitter at @brochman. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.